The present invention relates to a valve lift device for a combustion engine.
The inlet valves and exhaust valves of combustion engines are usually controlled by a rotating camshaft provided with cams which serve as guide surfaces for a cam follower. The cam followers therefore undergo substantially vertical lifting movements which are converted, via suitable motion-transmitting components, to lifting movements for the inlet valves and the exhaust valves. The opening and closing movements of the inlet valves and exhaust valves take place when the pistons in the combustion engine's respective cylinders are at fixed predetermined positions. The fixed positions for opening and closing the valves are a compromise arrived at to enable the engine to function well irrespective of its load and speed. The inlet valves and exhaust valves therefore do not always open and close at wholly optimum points in time in all operating conditions of the engine.
Controlling for example the closing time of the inlet valve may be advantageous from several points of view. Such control makes it possible to optimise the degree of filling of the cylinders at various different engine speeds, which is desirable when the engine is under heavy load. Controlling the inlet valve also allows control of the effective compression ratio. Postponing the inlet valve closing time relative to that which results in optimum degree of filling makes the compression begin later and therefore take place during a shorter proportion of the piston movement. The subsequent expansion does however remain unchanged. The result is that the expansion ratio is greater than the compression ratio, which in certain operating conditions is advantageous from an efficiency point of view. However, it is not possible to close the inlet valve late in all operating conditions. For example, when a combustion engine is being started up, the compression ratio would be so low that no ignition would take place.
A high exhaust temperature is often necessary to enable equipment for post-treatment of exhaust gases to work well. When there is low load upon the combustion engine, the air flow through it will be high relative to the amount of fuel supplied, resulting in a low exhaust temperature. The exhaust temperature may be raised by reducing the amount of air led to the engine. A throttle valve is normally used to reduce the amount of air led to the engine. However, using a throttle valve entails losses. Controlling the inlet valve closing time is an alternative way of controlling the air flow to the engine.
Control of the opening time of the exhaust valve may be used to raise the exhaust temperature. Opening the exhaust valve earlier than normal will end the expansion at a higher temperature, resulting in a raised exhaust temperature. In supercharged combustion engines, the exhaust turbine is so dimensioned as to be able to provide high charge pressure at low engine speed. This means that the turbine would over speed at high engine speed and load. To avoid this, part of the exhaust flow is led past the turbine through a so-called waste gate. The need for a waste gate may be reduced by postponing the opening time of the exhaust valve. This would also increase the efficiency.
In supercharged combustion engines, opening the exhaust valves early provides the exhaust turbine with more energy and consequent potential for higher charge pressure. Opening the exhaust valve late provides more energy to the engine, which therefore achieves greater efficiency. Variable exhaust valve opening times therefore make it possible to vary the efficiency and performance of the engine. During transients it may also be advantageous to open the exhaust valves later and thereby achieve a faster increase in charge air pressure.